The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
In an attempt to reduce sensor height, and thereby reduce bit length, researchers have recently been developing self pinned sensors. The pinned layer of a self pinned sensor achieves pinning by taking advantage of the high positive magnetostriction of certain magnetic materials. Sensors inevitably contain compressive stresses. By constructing an antiparallel pinned structure of such positive magnetostriction materials, these compressive stresses can be used to generate a strong magnetic anisotropy in the pinned layers. This anisotropy provides the pinning allowing the sensor to be built without an AFM layer. Since AFM layers are very thick relative to the other layers of the sensor, this leads to a significant reduction of sensor height.
In another attempt to increase sensor performance, researchers have developed dual GMR sensors. Such sensors include two free layers sharing a common pinned layer, or may include two pinned layers sharing a common free layer. Such devices have the potential to substantially increase GMR effect by doubling the number of spacer layers and the amount of spin dependent scattering.
In further attempts to increase data rate and data density, researchers have been focusing on the use of perpendicular recording. Conventional magnetic storage systems have recorded bits of data as magnetic transitions oriented longitudinally on the magnetic medium. As the name suggests, a perpendicular system records bits of data as magnetic transitions oriented perpendicular to the magnetic medium. The system employs a magnetic medium having a high coercivity top layer and a magnetically softer underlayer. The perpendicular recording system also employs a magnetic write element having a write pole with a small cross section and a return pole having a much larger cross section. Magnetic field from the write pole emits as a highly concentrated magnetic field oriented perpendicular to the surface of the medium. This concentrated field is sufficiently high to overcome the high coercivity of the upper layer of the magnetic medium and thereby write a magnetic transition thereon. The resulting flux in the medium then travels through the soft under layer, where it returns to the return pole in a more spread out pattern having insufficient strength to erase the signal from the high coercivity top layer.
Sensors can be categorized as current in plane (CIP), sensors wherein current flows through the sensor in a direction parallel with the plane of the layers making up the sensor (ie. side to side), or as current perpendicular to plane (CPP) sensors, wherein current flows perpendicular to the planes of the layers making up the sensor, (ie. from top to bottom). Sensors have traditionally been constructed as CIP sensors, however, the increased focus on perpendicular recording has lead to an increased interest in CPP sensors. This is because CPP sensors are more suited to use in perpendicular recording systems than are CIP sensors.
As can be seen, there has been much effort expended in developing recording systems that can provide ever increasing data rate and data density. Regardless of the scheme used to produce such advances, one overriding factor remains. In order to increase data rate and data density, magnetoresistive read elements must be constructed ever smaller. The smaller read elements must tolerate ever larger concentrations of electrical current which leads to ever increasing heat buildup in the sensor. The heat generated from the sense current then, becomes a limiting factor in increasing sensor performance and reducing sensor size, regardless of the advances made in other areas of design.
Therefore, there remains a strong felt need for a means for minimizing the heat build-up in a magnetoresistive sensor. Such a mechanism for reducing heat would preferably be suitable for use in a CPP sensor, since these sensors are of most interest in future perpendicular recording systems.